Two rabbits were immunized with multiple injections of recombinant preS1 peptide (rpreS1) utilizing the commercial Ribi Adjuvant System. Sera were screened for specific anti-rpreS1 by immune blotting and by radioimmunoassay; very little specific antibody was detected. Two additional rabbits were then immunized with the same immunogen in Freund's Adjuvant, and although a good response was observed, the antisera were not specific for preS1, suggesting that the rpreS1 immunogen was not pure. The anti-preS1 titers observed in these sera were lower than those of a previous preparation obtained by immunization with a synthetic preS1 peptide (residues 21-47) conjugated to keyhole limpet hemocyanin, and they were also lower than that of commercially available monoclonal anti-preS1. Because the rpreS1 preparation was recently found to contain two peptides, separable on 10% Tricine gels, we have now immunized two rabbits with the material from these separated bands. It is hoped that this will yield specific antibodies against preS1.